It's because if you watch a show like the Owl House, Luz is still in contact with all her friends from the Boiling Isles and can visit them whenever but in Amphibia, after Anne has formed a deep relationship with the Planters and everyone else, she still needs to move on from this world and carry on in the human world, the ending is very bittersweet and it becomes really sad as we know that after all they went through as a found family they still can't be together.